If affs_bread() fails, the exit path calls mark_buffer_dirty_inode() with a
NULL argument.

Coverity CID: 312.
Signed-off-by: NFlorin Malita <fmalita@gmail.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>